"If that fails (or your regex fails, or whatever validation you use), ____SUGGEST____ to the user that the address appears to be invalid. There's no reason for

"If that fails (or your regex fails, or whatever validation you use), ____SUGGEST____ to the user that the address appears to be invalid. There's no reason for an overzealous registration form to refuse to accept the user's actual e-mail address." Bingo. Help people, don't hinder them.